1. Field of the Invention
The present invention relates to a transmission for driving left and right axles of a vehicle comprising a pair of hydrostatic transmissions (to be referred to as HSTs), each of which is a fluid combination of a hydraulic pump and a hydraulic motor, wherein one of the HSTs is provided for speed change in advancing and reversing and the other is for steering.
2. Related Art
U.S. Pat. No. 4,782,650, for example, describes a vehicle provided with a pair of HSTs disposed in a lateral row and connected with each other, with left and right axles projecting laterally outward from its respective HST. Driving wheels are fixed onto utmost ends of the axles. The left and right axles are driven by changing the angles of slant of movable swash plates provided for the its respective left or right HST.
The left and right axles are driven at the same speed by the pair of HSTs when the vehicle is advancing or reversing in a straight line. They are driven at different speeds when turning.
However, the above mentioned conventional vehicle can advance or reverse straight only when the output rotational speeds of the pair of the HSTs are equal. Thus, equalization of the output speed of each of the HSTs is required. This equalization takes a long time during operation of the vehicle. Additionally, accuracy is required when manufacturing and assembling the components of each HST. If there is any difference in volume of the hydraulic pumps or motors between the HSTs, the feeling in turning left and right are different from each other, thereby making the vehicle difficult to control.
Furthermore, a speed changing operating tool, such as a speed change pedal, is connected to both output speed changing members, such as a movable swash plates, of the pair of HSTs. A steering operating tool, such as a steering wheel, is also connected to both members. This arrangement requires complicated linkages interposed between the speed changing operating tool and the both speed changing members and between the steering operating tool and both speed changing members, thereby increasing the manufacturing cost of the linkages and the space for disposal thereof in the vehicle.
If one HST is provided for speed change in traveling and another is provided for steering, each of the operating tools for speed changing and steering need only be connected with one output speed changing member of either of the HSTs, thereby simplifying the linkages between the operating tools and the speed changing members and reducing the manufacturing cost and the space required for disposal thereof.
However, the transmission for speed changing and steering must be capable of turning along a small radius if it is employed by a working vehicle like a mower tractor required to travel among trees. It also must be compact if it is provided for a small vehicle.
Furthermore, it is required to be capable of being operated for speed changing and steering as easily and sensitively as a passenger car.
It is also desirable that the transmission""s activity in turning corresponds to the traveling speed to improve the sensitivity of the steering operation. Moreover, it is desired for safety and efficiency of work that when the same degree of steering operation is employed, the vehicle provided with the transmission turns along a small radius when traveling slowly and turns along a large radius when traveling fast.
A transmission for steering and speed changing of a vehicle in accordance with the present invention comprises a first HST including a hydraulic pump and a hydraulic motor for speed changing for advancing and reversing and a second HST including a hydraulic pump and a hydraulic motor for steering. The first HST is driven by a prime mover. Output power of the first HST is transmitted into a pair of axles through a differential and is also transmitted into the second HST.
Since the transmission is so constructed, the pair of HSTs can be driven by the single prime mover, so that only one input means, for example, an input shaft, of the first HST is needed to receive the output power of the prime mover. Also, since speed changing is provided only by the single first HST, and steering is provided by only the single second HST, the oil circuits thereof can be simplified and the manufacturing cost can be reduced. The transmission does not need complicated equalization of output power between the two HSTs when advancing or reversing in a straight line as in conventional dual HST arrangements. Moreover, since the wheels connected to the axles are used for both traveling and steering, the vehicle can employ simple follower wheels such as casters, thereby having a small turning radius and reducing the manufacturing cost.
For braking the axles, a brake device can be provided on any rotational member in the transmitting system for speed changing in traveling between an input side of the first HST and the axles. This provides flexibility in choosing the optimal position for placement of the brake device in order to provide a compact transmission.
An output speed changing member of the first HST is connected with a speed changing operating tool provided on a vehicle, so that the rotary speed of the motor of the first HST is steplessly changeable by operation of the tool, whereby the traveling speed of the vehicle can be steplessly controlled. Also, the connection between the speed changing operating tool and the output speed changing means is simpler in comparison with the conventional connection between a steering operating tool and a pair of output speed changing members of two HSTs.
Regarding the transmitting system for steering, the second HST driven by the first HST has a pair of output means, which rotate in opposite directions by output of the motor of the second HST. The pair of output means are drivingly connected with the pair of axles. An output speed changing member of the second HST is connected with a steering operating tool provided on the vehicle, so that the rotary speed of the motor of the second HST is steplessly changeable by operation of the steering operating tool. The connection between the steering operating tool and the output speed changing means is simpler in comparison with the conventional connection between a steering operating tool and a pair of output speed changing members of two HSTs.
In this construction, when the steering operating tool is operated for turning, the motor of the second HST is driven so as to rotate the output means of the second HST in opposite directions. Accordingly, one of the axles is accelerated and the other is decelerated, so that the vehicle turns to the side of the decelerated axle.
Since the rotational direction of the pump of the second HST is reversed according to reversing the motor of the first HST when the vehicle travels in reverse, the turning direction of the vehicle can coincide with the same direction of operation of the steering operating tool in both cases of advancing and reversing. This arrangement therefore does not require a mechanism for coinciding the turning direction of the vehicle with the operational direction of the steering operating tool when in reverse. Thus, the vehicle employing the transmission can be steered as easily as a passenger car.
Also, since the rotary speed of the pump of the second HST is increased in proportion to that of the motor of the first HST, the replication of turning to the steering operation can be more sensitive in proportion to traveling speed. Thus, the vehicle can be nicely steered because it is prevented from delay in turning in relation to the steering operation. Moreover, the pump of the second HST for steering is stopped when the vehicle is stopped because the second HST is driven by output power of the first HST for speed changing. Thus, the vehicle is safe from unexpected start even if an operator touches the steering operating tool on the stopped vehicle.
With regard to the differential connected with the axles, it may constitute a pair of planetary gears. In this case, the output means of the second HST is drivingly connected with the pair of planetary gears. Due to this construction, the pair of planetary of gears as the differential can be narrowed and made more compact because of omission of a differential casing.
With regard to the connection between the steering operating tool and the output speed changing member of the second HST, it may be constructed such that the member is moved at a small rate of speed relative to an operational degree of the tool when the member is in vicinity of its neutral position. This construction allows the vehicle to travel straight even if the steering operating tool is slightly moved from its straight traveling position, and it turns gradually on steering of the steering operation for turning.
For the purpose of providing a changeable turning circle in correspondence to traveling speed, the transmission employs either of the following two constructions.
A first linkage, which has a pair of fixed pivotal points and an intermediate movable pivotal point between the fixed pivotal points, is interposed between the steering operating tool and the output speed changing member of the second HST. A second linkage is interposed between the speed changing operating tool and the movable pivotal point. The first linkage is bendable at the movable pivotal point thereof in correspondence to the steering operation. The second linkage moves the movable pivotal point in correspondence to the speed changing operation so as to change a ratio of a distance between one fixed pivotal point and the movable pivotal point to that between the other fixed pivotal point and the movable pivotal point, whereby the output rotational speed of the second HST is reduced in inverse proportion to the traveling speed.
Alternatively, the pump of the first HST is of variable capacity type and the motor thereof is of fixed capacity type and both of the pump and the motor of the second HST are of variably capacity types. A first capacity changing means of the pump of the first HST is connected with a second capacity changing means of the motor of the second HST, so that the rotary speed of the motor is reduced in inverse proportion to that of the pump.
In either construction, when the steering operating tool is operated to the same degree, the turning radius of the vehicle is large when traveling fast and is small when traveling slowly. The vehicle is safe from sudden turning when traveling fast, and it can turn rapidly by small steering operation when traveling slowly.
With regard to disposal of the first and second HSTs in relation to the housing containing the differential, the axles and the gear trains between the differential and the axles therein, the HSTs may be provided outside the housing, thereby enabling the HSTs to be easily attached and removed or assembled and disassembled independently of the settled housing.
Alternatively, the first and second HST may be contained within the housing, thereby providing a more compact, lighter transmission with reduced manufacturing cost in comparison with that provided with other housings for the HSTs.
A third option is to provide either of the HSTs outside the housing and the other within the housing, whereby the transmission can be constructed easily and cheaply by using a common axle driving apparatus comprising an HST, a differential and axles contained in a housing and using another common HST for the externally provided HST.
These and other objects, features and advantages of the invention will become more apparent in the detailed description and examples which follow.